Types Of Composites

Composites can be divided into two general groups of organic compounds: thermoplastic and thermoset.

Thermoplastic Thermoplastic materials can be repeatedly melted and solidified. They do not go through an irreversible chemical process when heated. Amorphous thermoplastics resins have a broad softening range and are suitable for processes that require good melting strength like thermoforming. They can be transparent in color but do have limited chemical resistance.

Crystalline thermoplastic resins have a sharp melt point and are better suited to molding processes like injection molding where melt strength is not required. They have excellent chemical resistance. Their physical properties can be modified and improved with the addition of glass or carbon fiber, tougheners, or mica. Chart 1 shows the effect of various reinforcement materials on selected polymers.

Some of the thermoplastic processes that are used to form pump parts are injection molding, vacuum forming, extrusion, and blow molding. Thermoplastic resins suitable for pump applications include floropolymers, acrylics, polyethylene, polypropylene, polyvinyl chloride, and others. The allowable temperature range for these materials is from

CHART 1 Thermoplastics strength versus reinforcement

' % OF REINFORCEMENT

CHART 1 Thermoplastics strength versus reinforcement

CONTINUOUS SERVICE TEMPERATURE 'F FIGURE 1 The effect of temperature on modules of elasticity

130-300°F (54-150°C), depending on the material. Figure 1 shows the elastic modulus versus service temperature for various polymers.

Thermoset Thermoset materials, when heated, go through an irreversible chemical change with the result that the material will not soften when reheated. Thermoset resins are generally reinforced with glass fiber, linen, carbon fiber, graphite, minerals and so on.

5.2 MATERIALS OF CONSTRUCTION FOR NONMETALLIC (COMPOSITE) PUMPS Differences between Thermoset and Thermoplastic

They have an allowable application temperature range of 180-250°F (82-120°C), depending on the material. Examples of thermosets are epoxies and vinyl esters. They can be stronger than thermoplastics but are usually more brittle. Thermosets are processed by comparison molding or transfer molding. They have good chemical resistance to acids, bases, and oxidizing agents.

Table 1 outlines the differences between thermoset and thermoplastic polymers.